Liquid crystalline medium

ABSTRACT

The invention relates to a liquid crystalline medium, based on a mixture of polar compounds with a positive dielectric anisotropy. Said medium is characterised in that it contains one or more compounds of formula (I), wherein R, A1, A2, Z1, Z2, X1, X2, n and o are defined as per claim 1.

[0001] The present invention relates to a liquid-crystalline medium, tothe use thereof for electro-optical purposes, to displays containingthis medium, and to novel compounds for use in the liquid-crystallinemedium according to the invention.

[0002] Liquid-crystals are used principally as dielectrics in displaydevices, since the optical properties of such substances can be modifiedby an applied voltage. Electro-optical devices based on liquid crystalsare extremely well known to the person skilled in the art and can bebased on various effects. Examples of such devices are cells havingdynamic scattering, DAP (“deformation of aligned phases”) cells,guest/host cells, TN cells having a twisted nematic structure, STN(“supertwisted nematic”) cells, SBE (“superbirefringence effect”) cells,OMI (“optical mode interference”) cells, OCB (“optically compensatedbend mode”) and IPS (“in-plane-switching”) cells.

[0003] The commonest display devices are based on the Schadt-Helfricheffect and have a twisted nematic structure, wie z.B. in TN-undSTN-Zellen. Sie können als Multiplex-oder als Aktivmatrix-Anzeigen(AMD-TN, AMD=active matrix driven) betrieben werden.

[0004] In the case of TN displays, liquid-crystal media are desiredwhich enable the following advantages in the cells: an expanded nematicphase range (in particular down to low temperatures), switchability atextremely low temperatures (outdoor use, automobiles, avionics) andincreased resistance to UV radiation (longer service life). With themedia available from the prior art, however, it is not possible toachieve these advantages while simultaneously retaining the aboveparameters.

[0005] In the case of the more highly twisted STN displays,liquid-crystal media are desired which enable greater multiplexabilityand/or lower threshold voltages and/or broader nematic phase ranges (inparticular at low temperatures). To this end, a further extension of theavailable parameter latitude (clearing point, smectic-nematic transitionor melting point, viscosity, dielectric parameters, elastic parameters)is urgently desired.

[0006] OCB displays contain a liquid-crystal layer having a so-called“bend” structure. The “bend” cell, also known as “π” cell, was proposedfor the first time by P. Bos et al., SID 83 Digest, 30 (1983), forelectrically controllable λ/s retardation plates. Optical displays basedon the OCB cell have been described by Y. Yamaguchi, T. Miyashita and T.Uchida, SID 93 Digest, 277 (1993), T. Miyashita et al. in Proc.Eurodisplay, 149 (1993), J. Appl. Phys. 34, L177 (1995), SID 95 Digest,797 (1995), and C. -L. Kuo et al., SID 94 Digest, 927 (1994). OCB cellsusually contain a liquid-crystal layer having a homogeneous edgealignment (i.e. parallel to the surfaces) and positive dielectricanisotropy. In addition, the OCB displays disclosed in theabove-mentioned documents usually have one or more optical retardationfilms in order to reduce undesired light transmission of the “bend” cellin the dark state. OCB displays have some advantages over conventionalTN cells, such as, for example, a wider viewing angle and shorterresponse times.

[0007] OCB displays require liquid-crystalline media which have tosatisfy a multiplicity of requirements. Particularly important here arethe chemical resistance to moisture, air and physical influences, suchas heat, radiation in the infrared, visible and ultraviolet region, anddirect and alternating electric fields. Furthermore, liquid-crystalmedia for the OCB effect which can be used industrially are required tohave a liquid-crystalline phase in a suitable temperature range,relatively high birefringence, positive dielectric anisotropy, arelatively low value for the ratio of the elastic constants K₃/K₁ andlow viscosity. There is thus a great demand for liquid-crystalline mediafor OCB displays which exhibit, in particular, high values for thebirefringence and dielectric anisotropy and at the same time lowviscosities.

[0008] Besides the known liquid-crystal displays (TN, STN, OMI, AMD-TNand OCB), in which the electric fields for realignment are generatedessentially perpendicular to the liquid-crystal layer, displays alsoexist in which the electric signals are generated in such a way that theelectric fields have a significant component parallel to theliquid-crystal layer. A display of this type, known as an IPS (“in-planeswitching”) display, is disclosed, for example, in international patentapplication WO 91/10936. The principles of operation of a display ofthis type are described, for example, by R. A. Soref in Journal ofApplied Physics, Vol. 45, No.12, pp. 5466-5468 (1974). For example, EP 0588 568 discloses various possibilities for the design of the electrodesand for addressing a display of this type. DE 198 24 137 likewisedescribes various embodiments of IPS displays of this type.Liquid-crystalline materials for IPS displays are described, forexample, in DE 195 28 104.

[0009] The IPS displays containing the known liquid-crystalline mediaare characterised by inadequate, long response times and often byexcessively high operating voltages. There is thus a demand forliquid-crystal media for IPS displays which do not have thesedisadvantages or only do so to a lesser extent. To this end, there is aparticular requirement for liquid-crystalline materials which, besidesan adequate phase range, a low tendency toward crystallisation at lowtemperatures, low birefringence and adequate electrical resistance,have, in particular, low threshold voltages (V₁₀) and low rotationalviscosities (γ₁), which are crucial for the response times.

[0010] Just like TN displays, OCB and IPS displays can also be operatedas matrix displays.

[0011] Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

[0012] 1. MOS (metal oxide semiconductor) or other diodes on a siliconwafer as substrate.

[0013] 2. Thin-film transistors (TFTs) on a glass plate as substrate.

[0014] The use of single-crystal silicon as substrate material restrictsthe display size, since even modular assembly of various part-displaysresults in problems at the joints.

[0015] In the case of the more promising type 2, which is preferred, theelectrooptical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. The latter technology is being worked on intensivelyworldwide.

[0016] The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is generallyarranged in such a way that a filter element is opposite each switchablepixel.

[0017] The TFT displays usually operate as TN cells with crossedpolarisers in transmission and are illuminated from the back. In thecase of OCB displays, reflective displays have also been proposed, forexample by T. Uchida, T. Ishinabe and M. Suzuki in SID 96 Digest, 618(1996).

[0018] The term MLC displays here covers any matrix display withintegrated non-linear elements, i.e., besides the active matrix, alsodisplays with passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

[0019] For matrix liquid-crystal displays having integrated non-linearelements for switching individual pixels (MLC displays), liquid-crystalmedia having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and low vapour pressure are desired.

[0020] MLC displays are particularly suitable for TV applications (forexample pocket TVs) or for high-information displays for computerapplications (laptops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due toinsufficiently high specific resistance of the liquid-crystal mixtures[TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, Sept. 1984:A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff,Paris; STROMER, M., Proc. Eurodisplay 84, Sept. 1984: Design of ThinFilm Transistors for Matrix Addressing of Television Liquid CrystalDisplays, p.145 ff, Paris]. With decreasing resistance, the contrast ofan MLC display deteriorates, and the problem of after-image eliminationmay occur. Since the specific resistance of the liquid-crystal mixturegenerally drops over the life of an MLC display owing to interactionwith the interior surfaces of the display, a high (initial) resistanceis very important in order to obtain acceptable service lives. Inparticular in the case of low-volt mixtures, it was hitherto impossibleto achieve very high specific resistance values. It is furthermoreimportant that the specific resistance exhibits the smallest possibledecrease with increasing temperature and after heating and/or UVexposure. The low-temperature properties of the mixtures from the priorart are also particularly disadvantageous. It is demanded that nocrystallisation and/or smectic phases occur, even at low temperatures,and the temperature dependence of the viscosity is as low as possible.The MLC displays from the prior art thus do not meet today'srequirements.

[0021] Besides liquid-crystal displays which use back lighting, i.e. areoperated transmissively and optionally transflectively, there is alsoparticular interest in reflective liquid-crystal displays. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than back-litliquid-crystal displays of corresponding size and resolution. Since theTN effect is characterised by very good contrast, reflective displays ofthis type are readily legible even under bright ambient conditions. Thisis already known of simple reflective TN displays, as used, for example,in wristwatches and pocket calculators. However, the principle can alsobe applied to high-quality, higher-resolution active matrix-addresseddisplays, such as, for example, TFT displays. Here, as is already thecase in the generally conventional transmissive TFT-TN displays, the useof liquid crystals of low birefringence (Δn) is necessary in order toachieve low optical retardation (d·Δn). This low optical retardationresults in a low viewing-angle dependence of the contrast, which isusually acceptable (cf. DE 30 22 818). In reflective displays, the useof liquid crystals of low birefringence is much more important than intransmissive displays, since in reflective displays, the effective layerthickness through which the light passes is approximately twice as greatas in transmissive displays of the same layer thickness.

[0022] There thus continues to be a great demand for MLC displays havingvery high specific resistance at the same time as a largeworking-temperature range, short response times even at low temperaturesand low threshold voltage which do not have these disadvantages, or onlydo so to a reduced extent.

[0023] In general, liquid-crystal materials for the above-mentioneddisplay types must have good chemical and thermal stability and goodstability towards electric fields and electromagnetic radiation.Furthermore, the liquid-crystal materials should have low viscosity andgive short addressing times, low threshold voltages and high contrast inthe cells.

[0024] Furthermore, they should have a suitable mesophase, for example anematic or cholesteric mesophase for the above-mentioned cells, at usualoperating temperatures, i.e. in the broadest possible range below andabove room temperature. Since liquid crystals are generally used in theform of mixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Other properties, suchas the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

[0025] None of the series of compounds having a liquid-crystalline phasethat have been disclosed hitherto contains a single compound which meetsall the above-mentioned requirements. In general, therefore, mixtures offrom 2 to 25, preferably from 3 to 18, compounds are prepared in orderto obtain substances which can be used as liquid-crystal phases.However, it has not been easy to prepare optimum phases in this way,since no liquid-crystal materials having high birefringence, positivedielectric anisotropy and high clearing point at the same time as lowrotational viscosity were available hitherto.

[0026] The invention has the object of providing liquid-crystallinemedia, in particular for the above-mentioned MLC, TN, STN, OCB and IPSdisplays, which meet the above-mentioned requirements, do not have theabove-mentioned disadvantages, or only do so to a lesser extent, andpreferably at the same time have very high specific resistance valuesand low threshold voltages.

[0027] It has now been found that the above-described objects can beachieved if media according to the invention are used in displays.

[0028] The invention thus relates to a liquid-crystalline medium basedon a mixture of polar compounds, characterised in that it comprises oneor more compounds of the formula I

[0029] in which

[0030] R is F, Cl, Br, I, CN, NCS, SF₅ or an alkyl radical having from 1to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃or monosubstituted or polysubstituted by halogen and in which one or twonon-adjacent CH₂ groups may be replaced by —O—, —CH═CH—, —CF═CF—, —C≡C—,—CO—, —OCO— or —COO— in such a way that O atoms are not linked directlyto one another,

[0031] A¹ and A² are each, independently of one another, 1,4-phenylene,in which, in addition, one or two CH groups may be replaced by N andwhich may also be monosubstituted or polysubstituted by L, or aretrans-1,4-cyclohexylene, in which, in addition, one or more non-adjacentCH₂ groups may be replaced by —O— and/or —S—, or are1,4-cyclohexenylene, 1,4-bicyclo-[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl or1,2,3,4-tetrahydro-naphthalene-2,6-diyl,

[0032] L is F, Cl, Br, I, CN, NCS, SF₅ or alkyl, alkoxy, alkylcarbonyl,alkylcarbonyloxy, alkoxycarbonyl, alkenyl or oxaalkenyl having from 1 to3 carbon atoms, in which, in addition, one or more H atoms may bereplaced by F or Cl,

[0033] Z¹ and Z² are each, independently of one another, —CH₂O—, —OCH₂—,—CF₂O—, —OCF₂—, —COO—, —OCO—, —CF₂CF₂—, —CH₂CH₂—, —(CH₂)₄—, —(CH₂)₃O—,—O(CH₂)₃—, —CF₂CH₂—, —CH═CH—, —CH═CF—, CF═CF—, —C═C— or a single bond,

[0034] X¹ and X² are each, independently of one another, F, Cl, Br, I,CN, NCS, SF₅ or alkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy,alkoxycarbonyl, alkenyl or oxaalkenyl having up to 5 carbon atoms, inwhich, in addition, one or more H atoms may be replaced by F or Cl, andone of the radicals X¹ and X² is alternatively H or R,

[0035] n and o are each, independently of one another, 0, 1 or 2, wheren+o is ≦3.

[0036] The compounds of the formula I have low rotational viscosities atthe same time as favourable clearing points and high dielectricanisotropy Δε, and effect a reduction in the threshold voltage in themedia according to the invention at the same time as an optimisation ofthe low-temperature behaviour. They are particularly suitable for use inTFT-TN displays with all common operating voltages (driver voltage of 5V, 4 V, 3.3 V and 2.5 V), in particular for TFT-TN displays having lowthreshold voltages (driver voltage of 2.5 and 3.3 V), and, owing totheir favourable birefringence values, for TN displays at the firstGooch and Tarry transmission minimum [C. H. Gooch and H. A. Tarry,Electron. Lett. 10, 2-4, 1974, and Appl. Phys., Vol. 8, 1575-1584,11975].

[0037] The compounds of the formula I have a broad range ofapplications. These compounds can be used as base materials of whichliquid-crystalline media are predominantly composed; however, it is alsopossible to add compounds of the formula I to liquid-crystalline basematerials from other classes of compound in order, for example, tomodify the dielectric and/or optical anisotropy of a dielectric of thistype and/or to optimise its threshold voltage and/or its viscosity.

[0038] In the pure state, the compounds of the formula I are colourlessand form liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light. The compounds of the formula I are prepared bymethods known per se, as described in the literature (for example in thestandard works, such as Houben-Weyl, Methoden der Organischen Chemie[Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to beprecise under reaction conditions which are known and suitable for thesaid reactions. Use can also be made here of variants which are knownper se, but are not mentioned here in greater detail. Some compounds ofthe formula I and their preparation are disclosed in DE 43 03 634 and DE44 09 526. The novel compounds of the formula I are a furthersubject-matter of the invention.

[0039] Particular preference is given to compounds of the formula I inwhich n+o is 0 or 1.

[0040] Preference is furthermore given to compounds of the formula I andtheir sub-formulae in which one or both radicals X¹ and X² are F, Cl, CNor fluorinated alkyl or alkoxy having from 1 to 3 carbon atoms, inparticular CF₃, OCF₃, CF₂H or OCF₂H, furthermore OCH₃ or OC₂H₅.

[0041] Preference is furthermore given to compounds of the formula I andtheir sub-formulae in which Z¹ and Z² are —OCO—, —COO—, —CF₂O—, —OCF₂—,—CH₂CH₂—, —CF₂CF₂— or a single bond.

[0042] Preference is furthermore given to compounds of the formula I andtheir sub-formulae in which R is alkyl or alkoxy having from 1 to 8carbon atoms.

[0043] The compounds of the following sub-formulae are particularlypreferred. In these, Cyc denotes a 1,4-cyclohexylene radical, which mayalso be substituted in the 1- and/or 4-position by F, Cl, CN or CF₃, Diodenotes a 1,3-dioxane-2,5-diyl radical, Phe denotes a 1,4-phenyleneradical, which may be substituted in the 2-, 3- and/or 5-position by L.Z has one of the meanings indicated for Z¹ in the formula I. InX¹X²denotes an indan-2-yl radical in accordance with the formula I which issubstituted in the 5-position by X¹ and in the 6-position by X². L, R,X¹ and X² are as defined in the formula I. R-Cyc-Z¹-InX¹X² I1R-Phe-Z¹-InX¹X² I2 R-Dio-Z¹-InX¹X² I3 R-Cyc-Z²-Phe-Z¹-InX¹X² I4R-Cyc-Z²-Cyc-Z¹-InX¹X² I5 R-Phe-Z²-Cyc-Z¹-lnX¹X² I6R-Phe-Z²-Phe-Z¹-InX¹X² I7 R-Cyc-Z²-Dio-Z¹-lnX¹X² I8R-Dio-Z²-Cyc-Z¹-InX¹X² I9 R-Dio-Z²-Dio-Z¹-InX¹X² I10R-Phe-Z²-Dio-Z¹-InX¹X² I11 R-Dio-Z²-Phe-Z¹-InX¹X² I12R-Cyc-Z²-Cyc-Z¹-Cyc-Z¹-InX¹X² I13 R-Cyc-Z²-Cyc-Z¹-Phe-Z¹-InX¹X² I14R-Phe-Z²-Cyc-Z¹-Cyc-Z¹-InX¹X² I15 R-Cyc-Z²-Phe-Z¹-Phe-Z¹-InX¹X² I16R-Phe-Z²-Phe-Z¹-Cyc-Z¹-InX¹X² I17 R-Phe-Z²-Phe-Z¹-Phe-Z¹-InX¹X² I18R-Cyc-Z²-Cyc-Z¹-Dio-Z¹-InX¹X² I19 R-Dio-Z²-Cyc-Z¹-Cyc-Z¹-InX¹X² I20R-Cyc-Z²-Dio-Z¹-Dio-Z¹-InX¹X² I21 R-Dio-Z²-Dio-Z¹-Cyc-Z¹-InX¹X² I22R-Dio-Z²-Dio-Z¹-Dio-Z¹-InX¹X² I23 R-Dio-Z²-Dio-Z¹-Phe-Z¹-InX¹X² I24R-Dio-Z²-Phe-Z¹-Phe-Z¹-InX¹X² I25 R-InX¹X² I26

[0044] Particular preference is given to compounds of the sub-formulaeI1, I2, I4, I5, I6 and I7 and I26.

[0045] Very particularly preferred compounds of the formula I areselected from the following sub-formulae Ia to Ie:

[0046] in which R, Z¹, Z², X¹ and X² are as defined in the formula I.

[0047] Particular preference is given to compounds of the formula Ia, Iband Id, in particular those in which X¹ and X² are F, Z¹ is a singlebond and R is alkyl or alkoxy having from 1 to 8 carbon atoms.

[0048] Preference is furthermore given to compounds of the formulae Iand Ia to Id in which

[0049] X¹ and X² are F, Cl, CN, CF₃, OCF₃, CF₂H or OCF₂H,

[0050] one of the radicals X¹ and X² is F, CN, Cl, CF₃ or OCF₃ and theother is H or alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl oralkylcarbonyloxy having from 1 to 8 carbon atoms,

[0051] Z¹ and Z² are a single bond,

[0052] one or both radicals Z¹ and Z² are —CF₂O—, —OCF₂— or —CF₂CF₂—,

[0053] R is alkyl or alkoxy having from 1 to 8 carbon atoms.

[0054] The term “fluorinated alkyl or alkoxy having from 1 to 3 carbonatoms” is preferably CF₃, OCF₃, CFH₂, OCFH₂, CF₂H, OCF₂H, C₂F₅, OC₂F₅,CFHCF₃, CFHCF₂H, CFHCFH₂, CH₂CF₃, CH₂CF₂H, CH₂CFH₂, CF₂CF₂H, CF₂CFH₂,OCFHCF₃, OCFHCF₂H, OCFHCFH₂, OCH₂CF₃, OCH₂CF₂H, OCH₂CFH₂, OCF₂CF₂H,OCF₂CFH₂, C₃F₇ or OC₃F₇, in particular CF₃, OCF₃, CF₂H, OCF₂H, C₂F₅,OC₂F₅, CFHCF₃, CFHCF₂H, CFHCFH₂, CF₂CF₂H, CF₂CFH₂, OCFHCF₃, OCFHCF₂H,OCFHCFH₂, OCF₂CF₂H, OCF₂CFH₂, C₃F₇ or OC₃F₇, particularly preferablyOCF₃ or OCF₂H.

[0055] Halogen is preferably F or Cl, in particular F.

[0056] If R is an alkyl radical and/or an alkoxy radical, this may bestraight-chain or branched. It is preferably straight-chain, has 2, 3,4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl,butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexyloxyor heptyloxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl,tridecyl, tetradecyl, pentadecyl, methoxy, octyloxy, nonyloxy, decyloxy,undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy.

[0057] Oxaalkyl is preferably straight-chain 2-oxapropyl(=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-,3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8-, or 9-oxadecyl.

[0058] If R is an alkyl radical in which one CH₂ group has been replacedby —CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has from 2 to 10 carbon atoms. Accordingly, it is inparticular vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-, -3- or -4-enyl, hex-1-, -2-, -3-, 4- or -5-enyl, hept-1-, -2-, -3-,-4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl, non-1-,-2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-, -4-, -5-,-6-, -7-, -8- or -9-enyl.

[0059] If R is an alkyl radical in which one CH₂ group has been replacedby —O— and one has been replaced by —CO—, these are preferably adjacent.These thus contain an acyloxy group —CO—O— or an oxycarbonyl group—O—CO. These are preferably straight-chain and have from 2 to 6 carbonatoms. Accordingly, they are in particular acetoxy, propionyloxy,butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl,propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl,2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)-propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

[0060] If R is an alkyl radical in which one CH₂ group has been replacedby unsubstituted or substituted —CH═CH— and an adjacent CH₂ group hasbeen replaced by CO or CO—O or O—CO, this may be straight-chain orbranched. It is preferably straight-chain and has from 4 to 13 carbonatoms. Accordingly, it is in particular acryloyloxymethyl,2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl,5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl,8-acryloyloxyoctyl, 9-acryloyloxynonyl, 1 0-acryloyloxydecyl,methacryoyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl,4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or9-methacryloyloxynonyl.

[0061] If R is an alkyl or alkenyl radical which is monosubstituted byCN or CF₃, this radical is preferably straight-chain. The substitutionby CN or CF₃ is in any desired position.

[0062] If R is an alkyl or alkenyl radical which is at leastmonosubstituted by halogen, this radical is preferably straight-chain,and halogen is preferably F or Cl. In the case of polysubstitution,halogen is preferably F. The resultant radicals also includeperfluorinated radicals. In the case of monosubstitution, the fluorineor chlorine substituent may be in any desired position, but ispreferably in the ω-position.

[0063] Compounds of the formula I which have wing groups R which aresuitable for polymerisation reactions are suitable for the preparationof liquid-crystalline polymers.

[0064] Compounds containing branched wing groups R may occasionally beof importance owing to better solubility in the conventionalliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

[0065] Compounds of the formula I having S_(A) phases are suitable forthermally addressed displays.

[0066] Branched groups of this type generally contain not more than onechain branch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

[0067] If R is an alkyl radical in which two or more CH₂ groups havebeen replaced by —O— and/or —CO—O—, this may be straight-chain orbranched. It is preferably branched and has from 3 to 12 carbon atoms.Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.

[0068] The invention also relates to electro-optical displays containingliquid-crystal media according to the invention, in particular TN, STN,OCB, IPS or MLC displays having two plane-parallel outer plates, which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance which is located in the cell. The inventionfurthermore relates to the use of the liquid-crystal media according tothe invention for electro-optical use.

[0069] The liquid-crystal mixtures according to the invention enable asignificant widening of the available parameter latitude. The achievablecombinations of clearing point, viscosity at low temperature, thermaland UV stability and dielectric anisotropy are thus far superior toprevious materials from the prior art.

[0070] The requirement for a high clearing point, a nematic phase at lowtemperature, high Δε and at the same time low viscosity has hithertoonly been achieved to an inadequate extent. Although mixtures knownhitherto have relatively high values for the clearing point and for Asas well as favourable birefringence, they still have inadequately lowvalues for the rotational viscosity γ₁.

[0071] Other mixture systems have comparable viscosities and Δε values,but only have clearing points in the region of 60° C.

[0072] The liquid-crystal mixtures according to the invention, whileretaining the nematic phase down to −20° C. and preferably down to −30°C., particularly preferably down to −40° C., enable clearing pointsabove 75° C., preferably above 80° C., particularly preferably above 85°C., simultaneously dielectric anisotropy values Δε of ≧5, preferably ≧7,and a high value for the specific resistance to be achieved, enablingexcellent STN and MLC displays to be obtained. In particular, themixtures are characterised by low operating voltages. The TN thresholdsare <2.0 V, preferably 1.8 V, particularly preferably <1.6 V, veryparticularly preferably <1.4 V.

[0073] It goes without saying that, through a suitable choice of thecomponents of the mixtures according to the invention, it is alsopossible for higher clearing points (for example above 110°) to beachieved at a higher threshold voltage or lower clearing points to beachieved at lower threshold voltages with retention of the otheradvantageous properties. At viscosities correspondingly increased onlyslightly, it is likewise possible to obtain mixtures having greater Δεand thus lower thresholds. The MLC displays according to the inventionpreferably operate at the first Gooch and Tarry transmission minimum [C.H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch andH. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975] are used, where,besides particularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German Patent 30 22 818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistances to be achieved using the mixtures accordingto the invention at the first minimum than in the case of mixturescomprising cyano compounds. Through a suitable choice of the individualcomponents and their proportions by weight, the person skilled in theart is able to set the birefringence necessary for a pre-specified layerthickness of the MLC display using simple routine methods.

[0074] The rotational viscosity γ₁ at 20° C. is preferably <160 mPa.s,particularly preferably <140 mPa.s. The nematic phase range ispreferably at least 90°, in particular at least 100°. This rangepreferably extends at least from −20° to +80°.

[0075] Measurements of the capacity holding ratio (HR) [S. Matsumoto etal., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SIDConference, San Francisco, June 1984, p. 304 (1984); G. Weber et al.,Liquid Crystals 5, 1381 (1989)] have shown that mixtures according tothe invention comprising compounds of the formula I exhibit asignificantly smaller decrease in the HR with increasing temperaturethan, for example, analogous mixtures comprising cyanophenylcyclohexanesof the formula

[0076] or esters of the formula

[0077] instead of the compounds of the formula I.

[0078] In addition, it has been found that mixtures according to theinvention comprising compounds of the formula I have a higher clearingpoint and higher Δε than analogous mixtures comprisingcyanophenylcyclohexanes of the above-mentioned formula. Compared withthe last-mentioned mixtures, the mixtures according to the inventionalso have a smaller Δn, which is advantageous for many applications, inparticular reflective and transflective applications.

[0079] The UV stability of the mixtures according to the invention isalso considerably better, i.e. they exhibit a significantly smallerdecrease in the HR on exposure to UV.

[0080] The proportion of the compounds of the formula I in the mixtureof the media according to the invention as a whole is preferably 2-55%,preferably 3-35% and particularly preferably 5-15%.

[0081] The individual compounds of the following formulae and theirsub-formulae which can be used in the media according to the inventionare either known or can be prepared analogously to the known compounds.

[0082] Preferred embodiments are indicated below:

[0083] The medium additionally comprises one or more compounds selectedfrom the group consisting of the general formulae II to VII:

[0084] in which the individual radicals have the following meanings:

[0085] R⁰ is n-alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl, eachhaving up to 9 carbon atoms,

[0086] Z³ is —COO—, —CF₂O—, —C₂F₄— or a single bond,

[0087] Z⁴ is —COO—, —CF₂O—, —C₂F₄— or —C₂H₄—,

[0088] X⁰ is F, Cl, halogenated alkyl, alkenyl or alkoxy having up to 6carbon atoms,

[0089] Y¹ and Y² are each, independently of one another, H or F, and

[0090] r is 0 or 1.

[0091] The compounds of the formula II are preferably selected from thefollowing group:

[0092] in which R⁰ and X⁰ are as defined above, R⁰ is particularlypreferably n-alkyl having from 1 to 8 carbon atoms or alkenyl havingfrom 2 to 7 carbon atoms, and X⁰ is particularly preferably F, Cl, CF₃,OCF₃ or OCHF₂;

[0093] The compounds of the formula IV are preferably selected from thefollowing group:

[0094] in which R⁰, X⁰ and Y¹ are as defined above. R⁰ is particularlypreferably n-alkyl having from 1 to 8 carbon atoms or alkenyl havingfrom 2 to 7 carbon atoms. X⁰ is particularly preferably OCF₃, OCHF₂ orF. Particular preference is given to compounds of the formulae IVa, IVb,IVc, IVd and IVp;

[0095] The compounds of the formula VI are preferably selected from thefollowing group:

[0096] in which R⁰ and X⁰ are as defined above. R⁰ is particularlypreferably n-alkyl having from 1 to 8 carbon atoms or alkenyl havingfrom 2 to 7 carbon atoms. X⁰ is preferably OCF₃, OCHF₂ or F. Particularpreference is given to compounds of the formulae VIa, VIb, VIc, VIh, VIiand VIk;

[0097] The compounds of the formula VII are preferably selected from thefollowing group:

[0098] in which R⁰ and X⁰ are as defined above. R⁰ is particularlypreferably n-alkyl having from 1 to 8 carbon atoms or alkenyl havingfrom 2 to 7 carbon atoms. X⁰ is preferably OCF₃ or F, particularlypreferably F. Particular preference is given to the compounds of theformulae VIIa and VIIb;

[0099] The medium additionally comprises one or more compounds selectedfrom the following group:

[0100] in which R⁰, X⁰, Y¹ and Y² each, independently of one another,have one of the meanings indicated above. Y³ is H or F, and X⁰ ispreferably F, Cl, CF₃, OCF₃, OCHF₂, alkyl, oxaalkyl, fluoroalkyl oralkenyl, each having up to 6 carbon atoms.

[0101] Particular preference is given to compounds of the formula VIII,in particular those in which Y¹, Y², Y³ and X⁰ are F.

[0102] The medium comprises further compounds, preferably selected fromthe following group:

[0103] in which R⁰ and X⁰ are as defined above, and the 1,4-phenylenerings may be substituted by CN, chlorine or fluorine. The 1,4-phenylenerings are preferably monosubstituted or polysubstituted by fluorineatoms. In the compound XVIII, X⁰ is particularly preferably F or Cl.

[0104] The medium additionally comprises one or more-compounds selectedfrom the following group:

[0105] in which X³ has one of the meanings of X⁰ or is CN or NCS, andR⁰, X⁰, Y¹, Y² and r are as defined above.

[0106] Particularly preferred sub-formulae from the group consisting ofthe compounds of the formulae XIX and XX are the following:

[0107] in which R⁰ is as defined above and is particularly preferablyn-alkyl having from 1 to 8 carbon atoms or alkenyl having from 2 to 7carbon atoms.

[0108] The medium preferably comprises one or more dioxanes of theformulae D1 and D2:

[0109] in which R⁰ is as defined above.

[0110] The medium additionally comprises one or more compounds selectedfrom the following group:

[0111] in which R⁰ and Y¹ are as defined above, and R¹ and R² are each,independently of one another, alkyl or alkoxy having from 1 to 8 carbonatoms or alkenyl having from 2 to 7 carbon atoms.

[0112] In the compounds of the formulae XXII to XXVII, R¹ and R² arepreferably alkyl or alkoxy having from 1 to 8 carbon atoms.

[0113] Particular preference is given to the compounds selected from thefollowing group:

[0114] in which R^(1a) and R^(2a) are each, independently of oneanother, H, CH₃, C₂H₅ or n-C₃H₇, alkyl is an alkyl group having from 1to 7 carbon atoms, s is 0 or 1, and L is H or F.

[0115] R⁰ is straight-chain alkyl or alkenyl having from 2 to 7 carbonatoms;

[0116] The medium comprises compounds selected from the formulae II,III, IV, V, VI, VII, VIII, XIX, XX, XXI, XXII, XIII, XXIV and XXVII;

[0117] The medium comprises one or more compounds selected from theformulae IIa, IIb, IVa, IVb, IVc, IVd and VIk in which X⁰ is F,

[0118] The medium comprises one or more compounds selected from theformulae IIa, IVp, VIh and VIi in which X⁰ is OCF₃ or OCHF₂,

[0119] The I: (II+III+IV+V+VI+VII) weight ratio is preferably from 1:10to 10:1;

[0120] The medium essentially consists of compounds selected from thegroup consisting of the general formulae I to VIII and XIX to XXIV;

[0121] The medium comprises one, two or three compounds of the formulaI, preferably selected from the formulae Ia and Ic;

[0122] The proportion of compounds of the formula I in the mixture as awhole is from 2 to 55% by weight, in particular from 3 to 35% by weight,very particularly preferably from 5 to 15% by weight;

[0123] The proportion of compounds of the formulae I to VII together inthe mixture as a whole is at least 50% by weight;

[0124] The proportion of compounds of the formulae II to VII and XIX toXXVII in the mixture as a whole is from 30 to 95% by weight.

[0125] It has been found that even a relatively small proportion ofcompounds of the formula I mixed with conventional liquid-crystalmaterials, but in particular with one or more compounds of the formulaeII, III, IV, V, VI and/or VII, results in a significant lowering of thethreshold voltage and in low birefringence values, with broad nematicphases with low smectic-nematic transition temperatures being observedat the same time, improving the shelf life. Particular preference isgiven to mixtures which, besides one or more compounds of the formula I,comprise one or more compounds of the formula IV, in particularcompounds of the formula IVa, IVb, IVc and IVd in which X⁰ is F or OCF₃.The compounds of the formulae I to VII are colourless, stable andreadily miscible with one another and with other liquid-crystallinematerials.

[0126] The term “alkyl” covers straight-chain and branched alkyl groupshaving 1-7 carbon atoms, in particular the straight-chain groups methyl,ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbonatoms are generally preferred.

[0127] The term “alkenyl” covers straight-chain and branched alkenylgroups having 2-7 carbon atoms, in particular the straight-chain groups.Particular alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl. Examples ofpreferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

[0128] The term “fluoroalkyl” preferably covers straight-chain groupshaving a terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

[0129] The term “oxaalkyl” preferably covers straight-chain radicals ofthe formula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably=1 and m ispreferably from 1 to 6.

[0130] Through a suitable choice of the meanings of R⁰ and X⁰, theaddressing times, the threshold voltage, the steepness of thetransmission characteristic lines, etc., can be modified in the desiredmanner. For example, 1 E-alkenyl radicals, 3E-alkenyl radicals,2E-alkenyloxy radicals and the like generally result in shorteraddressing times, improved nematic tendencies and a higher ratio of theelastic constants K₃ (bend) and K₁ (splay) compared with alkyl or alkoxyradicals. 4-alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and smaller values of K₃/K₁ compared withalkyl and alkoxy radicals.

[0131] A —CH₂CH₂— group generally results in higher values of K₃/K₁compared with a single covalent bond. Higher values of K₃/K₁ facilitate,for example, flatter transmission characteristic lines in TN cells witha 90° twist (in order to achieve grey shades) and steeper transmissioncharacteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

[0132] The optimum mixing ratio of the compounds of the formulae I andII+III+IV+V+VI+VII depends substantially on the desired properties, onthe choice of the components of the formulae I, II, III, IV, V, VIand/or VII, and the choice of any other components that may be present.Suitable mixing ratios within the range given above can easily bedetermined from case to case.

[0133] The total amount of compounds of the formulae I to XXVII in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimising various properties. However, the observed effect on theaddressing times and the threshold voltage is generally greater, thehigher the total concentration of compounds of the formulae I to XXVII.

[0134] In a particularly preferred embodiment, the media according tothe invention comprise compounds of the formulae II to VII (preferablyII, III and/or IV, in particular IVa) in which X⁰ is F, OCF₃, OCHF₂, F,OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergistic effect with thecompounds of the formula I results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaI and of the formula IVa, IVb, IVc and IVd are distinguished by theirlow threshold voltages.

[0135] The construction of the MLC display according to the inventionfrom polarisers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term “conventional construction” is broadly drawn here and alsocovers all derivatives and modifications of the MLC display, inparticular including matrix display elements based on poly-Si TFT orMIM.

[0136] A significant difference between the displays according to theinvention and the conventional displays based on the twisted nematiccell consists, however, in the choice of the liquid-crystal parametersof the liquid-crystal layer.

[0137] The liquid-crystal mixtures which can be used in accordance withthe invention are prepared in a manner conventional per se. In general,the desired amount of the components used in the lesser amount isdissolved in the components making up the principal constituent,advantageously at elevated temperature. It is also possible to mixsolutions of the components in an organic solvent, for example inacetone, chloroform or methanol, and to remove the solvent again, forexample by distillation, after thorough mixing.

[0138] The dielectrics may also comprise further additives known to theperson skilled in the art and described in the literature. For example,0-15% of pleochroic dyes or chiral dopants can be added.

[0139] In the following examples, C denotes a crystalline phase, S asmectic phase, S_(C) a smectic C phase, N a nematic phase and I theisotropic phase. V₁₀ denotes the voltage for 10% transmission (viewingangle perpendicular to the plate surface). t_(on) denotes the switch-ontime and t_(off) the switch-off time at an operating voltagecorresponding to 2 times the value of V₁₀. Δn denotes the opticalanisotropy and n_(o) the refractive index. Δε denotes the dielectricanisotropy (Δε=ε_(∥)ε_(⊥), where ε_(⊥) denotes the dielectric constantparallel to the longitudinal molecular axes and ε_(⊥) denotes thedielectric constant perpendicular thereto). The electro-optical datawere measured in a TN cell at the 1st minimum (i.e. at a d·Δn value of0.5) at 20° C., unless expressly stated otherwise. γ₁ denotes therotational viscosity.

[0140] The above-mentioned data were measured at 20° C., unlessexpressly stated otherwise.

[0141] In the present application and in the examples below, thestructures of the liquid-crystal compounds are indicated by means ofacronyms, the transformation into chemical formulae taking place inaccordance with Tables A and B below. All radicals C_(n)H_(2n+1) andC_(m)H_(2m+1) are straight-chain alkyl radicals having n and m carbonatoms respectively. The coding in Table B is self-evident. In Table A,only the acronym for the parent structure is indicated. In individualcases, the acronym for the parent structure is followed, separated by adash, by a code for the substituents R¹, R², L¹ and L²: Code for R¹, R²,L¹, L² R¹ R² L¹ L² nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1)OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1)CN H H nN.F C_(n)H_(2n+1) CN H F nF C_(n)H_(2n+1) F H H nOFOC_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nF.F C_(n)H_(2n+1) F H FnF.F.F C_(n)H_(2n+1) F F F nCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₂ C_(n)H_(2n+1) OCHF₂ H H nS C_(n)H_(2n+1)NCS H H rVsN C_(r)H_(2r+1)—CH═CH— CN H H C_(s)H_(2s)— rEsNC_(r)H_(2r+1)—O—C₂H_(2s)— CN H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) H HnOCCF₂.F.F C_(n)H_(2n+1) OCH₂CF₂H F F

[0142] Preferred mixture components are given in Tables A and B. TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-nF

CGG

CGU

CUP

CCGU

[0143] TABLE B

CBC-nmF

PCH-nOm FET-nCl

CP-nOCF₃ CCH-nOm

BCH-n.Fm

Inm

CBC-nmF

ECCP-nm CCH-n1EM

T-nFm CGU-n-F

CCP-nOCF₃.F CGG-n-F

CCP-nOCF₂.F(.F) CCP-nF.F.F

CGU-n-OXF CUZU-n-F

CGU-n-O1DT CCZU-n-F

CC-n-V1 CC-n-V

CCP-nOCF₃ BCH-nF.F.F

CWCQU-n-F

CCOC-n-m

CGZU-n-F CUZP-n-F

CGU-1V-F CCG-V-F

CGZP-n-F UZP-n-N

CGZP-n-OT

CUZP-n-OT

CCQU-n-F Dec-U-n-F

Nap-U-n-F

CQGZP-n-F

CCQP-n-S

CPUQU-n-F

CCEEU-n-F CEECU-n-F

IS-5501 IS-5643

IS-5930 IS-5570

IS-5364 IS-5083

IS-5840

[0144] TABLE C Table C indicates possible dopants which are generallyadded to the mixtures according to the invention.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

CN

R/S-4011

R/S-2011

[0145] TABLE D Stabilisers which can be added, for example, to themixtures according to the invention are shown below.

[0146] The following examples are intended to explain the inventionwithout limiting it. Above and below, percentages are per cent byweight. All temperatures are indicated in degrees Celsius. m.p. denotesmelting point, cl.p. indicates clearing point. Furthermore,C=crystalline state, N=nematic phase, S=smectic phase and I=isotropicphase. The data between these symbols represent the transitiontemperatures. An denotes optical anisotropy (589 nm, 20° C.). Therotational viscosity γ₁ (mPa.s) was determined at 20° C.

EXAMPLE 1

[0147] A liquid-crystalline medium comprising CCZU-2-F 5.00% Clearingpoint [° C.]: 65.5 CCZU-3-F 14.00%  S-N transition [° C.]: <−40 CCZU-5-F4.00% Δn [589 nm, 20° C.]: 0.0909 BCH-3F.F.F 7.00% γ1 [mPa · s]: 139CGU-2-F 10.00%  V_((10,0,20)) [V]: 1.15 CGU-3-F 10.00%  CGU-5-F 5.00%CCP-20CF₃ 6.00% CCP-30CF₃ 8.00% CCP-2F.F.F 10.00%  CCP-3F.F.F 9.00%CC-1V-V1 4.00% IS-5501 8.00%

EXAMPLE 2

[0148] A liquid-crystalline medium comprising PCH-5F 3.59% Clearingpoint [° C.]: 97.2 CCP-20CF2.F.F 19.12%  Δε [1 kHz, 20° C.]: +8.2CCP-30CF2.F.F 17.95%  V_((10,0,20)) [V]: 1.38 CCP-50CF2.F.F 19.12% CUP-2F.F 6.01% CUP-3F.F 6.01% CBC-33F 6.01% CBC-53F 6.01% CBC-55F 5.92%IS-5501 10.24% 

EXAMPLE 3

[0149] A liquid-crystalline medium comprising CCZU-2-F 5.00% Clearingpoint [° C.]: 70.2 CCZU-3-F 15.00%  CCZU-4-F 5.00% CGU-2-F 10.00% CGU-3-F 10.00%  CGU-5-F 6.00% BCH-3F.F.F 5.00% CCP-2F.F.F 10.00% CCP-20CF₃ 9.00% CCZG-2-OT 10.00%  CCZG-3-OT 5.00% CC-3-V1 4.00% IS-55016.00%

EXAMPLE 4

[0150] A liquid-crystalline medium comprising PCH-5F 8.50% Clearingpoint [° C.]: 83.0 PCH-6F 6.80% Δε [1 kHz, 20° C.]: +6.0 PCH-7F 5.10% K₁[pN, 20° C.]: 12.2 CCP-20CF₃ 6.80% K₃/K₁: 1.41 CCP-30CF₃ 10.20% CCP-40CF₃ 6.00% CCP-50CF₃ 9.30% BCH-3F.F 10.30%  BCH-5F.F 8.50%ECCP-30CF₃ 4.20% ECCP-50CF₃ 4.20% CBC-33F 1.70% CBC-53F 1.70% CBC-55F1.70% IS-5501 5.00% IS-5643 5.50% IS-5364 4.50%

EXAMPLE 5

[0151] A liquid-crystalline medium comprising PCH-5F 9.00% Clearingpoint [° C.]: 87.0 PCH-6F 7.20% Δn [589 nm, 20° C.]: 0.0953 PCH-7F 5.40%Δε [1 kHz, 20° C.]: +5.8 CCP-20CF₃ 7.20% CCP-30CF₃ 10.80%  CCP-40CF₃6.30% CCP-50CF₃ 9.90% BCH-3F.F 10.80%  BCH-5F.F 9.00% ECCP-30CF₃ 4.50%ECCP-50CF₃ 4.50% CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% IS-550110.00% 

EXAMPLE 6

[0152] A liquid-crystalline medium comprising PCH-7F  6.00% Clearingpoint [° C.]: 93.0 CCP-20CF₃ 11.00% Δn [589 nm, 20° C.]: 0.0943CCP-30CF₃ 12.00% V_((10,0,20)) [V]: 1.55 CCP-40CF₃ 10.00% CCP-50CF₃12.00% BCH-3F.F.F 12.00% BCH-5F.F.F 11.00% CCP-3F.F.F 12.00% CCP-5F.F.F 9.00% IS-5501  5.00%

EXAMPLE 7

[0153] A liquid-crystalline medium comprising CCP-20CF₃ 11.00% Clearingpoint [° C.]: 98.0 CCP-30CF₃ 12.00% Δn [589 nm, 20° C.]: 0.0983CCP-40CF₃ 10.00% V_((10,0,20)) [V]: 1.50 CCP-50CF₃ 12.00% BCH-3F.F.F12.00% BCH-5F.F.F 11.00% CCP-3F.F.F 12.00% CCP-5F.F.F  9.00% IS-550111.00%

EXAMPLE 8

[0154] A liquid-crystalline medium comprising CCP-20CF₃ 11.00% Clearingpoint [° C.]: 71.4 CCP-30CF₃ 12.00% CCP-40CF₃ 10.00% CCP-50CF₃ 12.00%BCH-3F.F.F 12.00% BCH-5F.F.F 11.00% CCP-3F.F.F 12.00% CCP-5F.F.F  9.00%IS-5501  5.00% IS-5364  6.00%

EXAMPLE 9

[0155] A liquid-crystalline medium comprising PCH-5F 9.00% Clearingpoint [° C.]: 87.0 PCH-6F 7.20% Δn [589 nm, 20° C.]: 0.0951 PCH-7F 5.40%Δε [1kHz, 20° C.]: +5.7 CCP-20CF₃ 7.20% γ1 [mPa · s, 20° C.]: 121CCP-30CF₃ 10.80%  CCP-40CF₃ 6.30% CCP-50CF₃ 9.90% BCH-3F.F 10.80%BCH-5F.F 9.00% ECCP-30CF₃ 4.50% ECCP-50CF₃ 4.50% CBC-33F 1.80% CBC-53F1.80% CBC-55F 1.80% IS-5364 10.00% 

EXAMPLE 10

[0156] A liquid-crystalline medium comprising PCH-5F  5.00% Clearingpoint [° C.]: 90.8 CCP-20CF₃ 11.00% CCP-30CF₃ 12.00% CCP-40CF₃ 10.00%CCP-50CF₃ 12.00% BCH-3F.F.F 12.00% BCH-5F.F.F 11.00% CCP-3F.F.F 12.00%CCP-5F.F.F  9.00% IS-5364  6.00%

EXAMPLE 11

[0157] CC-3-V1 10.00%  Clearing point [° C.]: +80.0 CC-5-V 10.00%  Δn[589 nm, 20° C.]: +0.1033 CCH-35 5.00% d · Δn [μm, 20° C.]: 0.50 IS-55018.00% Twist [°]: 90 IS-5643 4.00% V_(10,0,20) [V]: 1.37 CCP-20CF₃ 8.00%CCP-30CF₃ 3.00% CCP-2F.F.F 3.00% PGU-2-F 8.00% PGU-3-F 8.00% CGZP-2-OT11.00%  CGZP-3-OT 9.00% CCZU-2-F 4.00% CCZU-3-F 9.00%

EXAMPLE 12

[0158] CCH-35 4.00% S → N [° C.]: < −40.0 IS-5501 9.00% Clearing point[° C.]: +78.5 CCP-2F.F.F 10.00%  Δn [589 nm, 20° C.]: +0.1033 CCP-3F.F.F11.00%  d · Δn [μm, 20° C.]: 0.50 CCP-20CF₃.F 9.00% Twist [°]: 90CCP-20CF₃ 8.00% CCP-30CF₃ 8.00% CCP-40CF₃ 7.00% CCP-50CF₃ 4.00% CGU-2-F10.00%  CGU-3-F 11.00%  CGU-5-F 4.00% CCGU-3-F 5.00%

EXAMPLE 13

[0159] PCH-3 22.50%  Clearing point [° C.]: +111.0 K6 7.20% Δε: +9.8 K98.10% K₁ [pN]: 14.8 CCP-20CF₃ 4.50% K₃/K₁: 1.55 CCP-30CF₃ 4.50%CCP-40CF₃ 4.50% CCP-50CF₃ 4.50% ECCP-20CF₃ 4.50% ECCP-30CF₃ 4.50%ECCP-50CF₃ 4.50% ECCP-3F 4.50% ECCP-5F 4.50% CBC-33F 4.50% CBC-53F 3.60%CBC-55F 3.60% IS-5840 10.00% 

EXAMPLE 14

[0160] PCH-6F 7.20% Clearing point [° C.]: +93.0 PCH-7F 5.40% γ1: [20°C., mPa · s]: 140 CCP-20CF₃ 7.20% CCP-30CF₃ 10.80%  CCP-40CF₃ 6.30%PCH-5F 9.00% CCP-50CF₃ 9.90% BCH-3F.F 10.80%  BCH-5F.F 9.00% ECCP-30CF₃4.50% ECCP-50CF₃ 4.50% CBC-33F 1.80% CBC-53F 1.80% CBCF-55F 1.80%IS-5840 10.01% 

EXAMPLE 15

[0161] PCH-3 22.49%  Clearing point [° C.]: +100.6 K6 7.20% Δε: +9.8 K98.10% CCP-20CF₃ 4.50% CCP-30CF₃ 4.50% CCP-40CF₃ 4.50% CCP-50CF₃ 4.50%ECCP-20CF₃ 4.50% ECCP-30CF₃ 4.50% ECCP-50CF₃ 4.50% ECCP-3F 4.50% ECCP-5F4.50% CBC-33F 4.50% CBC-53F 3.60% CBC-55F 3.60% IS-5840 10.03% 

EXAMPLE 16

[0162] BCH-3F.F 10.80%  Clearing point [° C.]: +65.8 BCH-5F.F 9.00% γ1[20° C., mPa · s]: 91 ECCP-30CF₃ 4.50% d · Δn [20° C., μm]: 0.50ECCP-50CF₃ 4.50% Twist [°]: 90 CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80%PCH-6F 7.20% PCH-7F 5.40% CCP-20CF₃ 7.20% CCP-30CF₃ 10.80%  CCP-40CF₃6.30% CCP-50CF₃ 9.90% PCH-5F 9.00% IS-5083 9.97%

EXAMPLE 17

[0163] CCZU-2-F 3.00% Clearing point [° C.]: +70.5 CCZU-3-F 13.00%  Δn:+0.0772 CCP-20CF₃ 4.00% γ1 [20° C., mPa · s]: 54 CCP-30CF₃ 8.00% d · Δn[20° C., μm]: 0.50 CGZP-2-OT 7.00% Twist [°]: 90 CGZP-3-OT 6.00% V₁₀[V]: 1.73 PGU-2-F 5.00% IS-5083 7.00% CC-5-V 10.00%  CC-3-V1 12.00% CCH-35 5.00% CC-3-V 18.00%  BCH-32 2.00%

EXAMPLE 18

[0164] CCZU-2-F 3.00% Clearing point [° C.]: +74.5 CCZU-3-F 13.00%  Δn:+0.0776 CCP-20CF₃ 7.00% γ1 [20° C., mPa · s]: 57 CCP-30CF₃ 6.00% d · Δn[20° C., μm]: 0.50 CGZP-2-OT 8.00% Twist [°]: 90 CGZP-3-OT 7.00% V₁₀[V]: 1.81 PGU-2-F 3.00% IS-5083 6.00% CC-5-V 7.00% CC-3-V1 13.00% CCH-35 5.00% CC-3-V 19.00%  BCH-32 3.00%

EXAMPLE 19

[0165] CCZU-2-F 3.00% CZU-3-F 13.00% CCP-20CF₃ 7.00% CGZP-2-OT 7.00%CGZP-3-OT 6.00% PGU-2-F 4.00% IS-5083 6.00% CC-5-V 10.00% CC-3-V1 11.00%CCH-35 5.00% CC-3-V 18.00% CBC-33 3.00%

EXAMPLE 20

[0166] PCH-3 20.00% K6 6.40% K9 7.20% CCP-20CF₃ 4.00% CCP-30CF₃ 4.00%CCP-40CF₃ 4.00% CCP-50CF₃ 4.00% ECCP-20CF₃ 4.00% ECCP-30CF₃ 4.00%ECCP-50CF₃ 4.00% ECCP-3F 4.00% ECCP-5F 4.00% CBC-33F 4.00% CBC-53F 3.20%CBC-55F 3.20% IS-5930 20.00%

EXAMPLE 21

[0167] PCH-6F 7.60% Clearing point [° C.]: +91.1 PCH-7F 5.70% γ₁ [20°C., mPa · s]: 141 CCP-20CF₃ 7.60% CCP-30CF₃ 11.40% CCP-40CF₃ 6.65%PCH-5F 9.50% CCP-50CF₃ 10.45% BCH-3F.F 11.40% BCH-5F.F 9.50% ECCP-30CF₃4.75% ECCP-50CF₃ 4.75% CBC-33F 1.90% CBC-53F 1.90% CBC-55F 1.90% IS-59305.01%

EXAMPLE 22

[0168] IS-5570 20.00% PCH-5F 3.20% CCP-30CF₂.F.F 16.00% CCP-50CF₂.F.F17.04% CUP-2F.F 5.36% CUP-3F.F 5.36% CBC-33F 5.36% CBC-53F 5.36% CBC-55F5.28% CCP-20CF₂.F.F 17.04%

1. Liquid-crystalline medium based on a mixture of polar compounds ofpositive dielectric anisotropy, characterised in that it comprises oneor more compounds of the formula I

in which R is F, Cl, Br, I, CN, NCS, SF₅ or an alkyl radical having from1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN orCF₃ or monosubstituted or polysubstituted by halogen and in which one ortwo non-adjacent CH₂ groups may be replaced by —O—, —CH═CH—, —CF═CF—,—C—C—, —CO—, —OCO— or —COO— in such a way that O atoms are not linkeddirectly to one another, A¹ and A² are each, independently of oneanother, 1 ,4-phenylene, in which, in addition, one or two CH groups maybe replaced by N and which may also be monosubstituted orpolysubstituted by L, or are trans-1,4-cyclohexylene, in which, inaddition, one or more non-adjacent CH₂ groups may be replaced by —O—and/or —S—, or are 1,4-cyclohexenylene, 1,4-bicyclo[2.2.2]octylene,piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diylor 1,2,3,4-tetrahydronaphthalene-2,6-diyl, L is F, Cl, Br, I, CN, NCS,SF₅ or alkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl,alkenyl or oxaalkenyl having from 1 to 3 carbon atoms, in which, inaddition, one or more H atoms may be replaced by F or Cl, Z¹ and Z² areeach, independently of one another, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—,—COO—, —OCO—, —CF₂CF₂—, —CH₂CH₂—, —(CH₂)₄—, —(CH₂)₃O—, —O(CH₂)₃—,—CF₂CH₂—, —CH═CH—, —CH═CF—, —CF═CF—, —C≡C— or a single bond, X¹ and X²are each, independently of one another, F, Cl, Br, I, CN, NCS, SF₅ oralkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl, alkenylor oxaalkenyl having up to 5 carbon atoms, in which, in addition, one ormore H atoms may be replaced by F or Cl, and one of the radicals X¹ andX² is alternatively H or R, n and o are each, independently of oneanother, 0, 1 or 2, where n+o is ≦3.
 2. Liquid-crystalline mediumaccording to claim 1, characterised in that it comprises one or morecompounds of the formulae Ia to Ie:

in which R, Z¹, X¹ and X² are as defined in claim
 1. 3.Liquid-crystalline medium according to claim 1 or 2, characterised inthat, in the formulae I, n+o is 0 or
 1. 4. Liquid-crystalline mediumaccording to one of claims 1 to 3, characterised in that, in the formulaI, X¹ and/or X² are F, Cl, CN, fluorinated alkyl or fluorinated alkoxy,each having from 1 to 3 carbon atoms.
 5. Liquid-crystalline mediumaccording to one of claims 1 to 5, characterised in that it additionallycomprises one or more compounds selected from the group consisting ofthe general formulae II, III, IV, V, VI and VII:

in which the individual radicals have the following meanings: R⁰ isn-alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl, each having up to 9carbon atoms, Z³ is —COO—, —CF₂O—, —C₂F₄— or a single bond, Z⁴ is —COO—,—CF₂O—, —C₂F₄— or —C₂H₄—, X⁰ is F, Cl, halogenated alkyl, alkenyl oralkoxy having from 1 to 6 carbon atoms, Y¹ and Y² are each,independently of one another, H or F. r is 0 or
 1. 6. Liquid-crystallinemedium according to one of claims 1 to 5, characterised in that itadditionally comprises one or more compounds selected from the groupconsisting of the general formulae VIII to XIV:

in which R⁰, X⁰, Y¹ and Y² are as defined in claim
 5. 7.Liquid-crystalline medium according to one of claims 1 to 6,characterised in that it additionally comprises one or more compoundsselected from the group consisting of the general formulae XXI to XXVII:

in which R⁰ and Y¹ are as defined in claim 5, and R¹ and R² are each,independently of one another, alkyl or alkoxy having from 1 to 8 carbonatoms or alkenyl having from 2 to 7 carbon atoms.
 8. Liquid-crystallinemedium according to one of claims 1 to 6, characterised in that theproportion of compounds of the formula I in the mixture as a whole isfrom 2 to 55% by weight.
 9. Liquid-crystalline medium according to oneof claims 2 to 7, characterised in that the proportion of compounds ofthe formulae I to VII in the mixture as a whole is at least 50% byweight.
 10. Use of the liquid-crystalline medium according to claim 1for electrooptical purposes.
 11. Electro-optical liquid-crystal displaycontaining a liquid-crystalline medium according to claim 1.